The invention of high energy density battery technologies initiated the market of cellular phones, notebook computers, electric cars, and grid storage applications. The market demand for smaller form factor and longer operating hours pushing the battery makers to invent higher and higher energy density battery cells. Higher energy density cells require more chemically active materials and inherently less stable and more difficult to design battery systems with high safety margins.
Most high energy density battery cells are consumed in cellular phones and portable computers. Total energy required for these devices are small, and relatively few battery cells, up to 8 of the 18650 form factor, are required. Battery life expectation for consumer products is typically 1-2 years. It is relatively easy to design a safe product with only a few battery cells.
However, battery pack for electric vehicles requires a lot of battery cells. A small electric car with 21 KW-hour capacity could have a driving range of 100 Kilo-meter, and would require 3,000 of the 18650 form factor battery cells, each with 7 watt-hour capacity. By necessity these battery cells must be packaged tightly together with only millimeter spacing, and could generate about 1,000-watt heat load while in operation. Without careful thermal design, battery cell temperature could elevate up to 30 Celsius above ambient, with detrimental effect on battery cell life. Battery system for vehicle operation is often 5 years or more, thermal management is a tough design issue for vehicle applications. Furthermore, high energy density battery cells become unstable when internal temperature exceeds 80 degree Celsius. It is cell chemistry and process dependent, and the probability of thermal run away, cell venting, and fire and explosion increases dramatically beyond such safe temperature limit. Battery pack must be design to not exceed this limit under any conditions.
When one battery cell goes into thermal runaway, either through violation of safe temperature limit, manufacturing process induced cell short circuit, over charge, or external impact from vehicle crashes, the amount of energy released may cause adjacent battery cells to also go into thermal runaway, this chain reaction destroys the battery pack and place the vehicle passengers in great physical danger. Prior art battery module designs relied on forced air cooling or liquid cooling to preserve pack safety. Unfortunately, cooling is often compromised in a vehicle crash, and damaged battery cells could spark thermal runaway that leads to explosion and vehicle fire. A high thermal conductivity battery module could dissipate the thermal energy from a runaway cell, lower the probability of fire and explosion, and minimize the thermal impact to adjacent cells, and prevent the inception of a chain reaction, without relying on a working cooling system.
Battery is typically constructed by rolling a sandwich of anode/separator/cathode in a sheet form into a jelly roll for cylindrical cells with superior thermal conduction in the same direction as the conductive anode/cathode sheets. It is due to the fact that anode/cathode sheets are constructed with metal with good thermal and electrical conductivity. The positive and negative connections are brought out in either the top plane or bottom plane in the same direction as the jelly roll. In a direction perpendicular to the sheets, thermal conduction is significantly worse, because heat must traverse metal, non-conductive separator, metal, non-conductive separator, several times before reaching the outer edge. For prismatic battery cells, it is typically constructed by a flattened version of a jelly roll or an interleaved anode/separator/cathode structure that also exhibit the same characteristics in thermal conduction. For pouch cells, construction is similar to prismatic battery cells except the outer enclosure is a soft pouch.
In other words, even though the distance between the top surface and the bottom surface is greater than the one between the opposite sides, thermal conductivity for the battery cell toward its top face and bottom surface is much better than toward the lateral sidewall. A factor of 12 or more in thermal conductance difference between top/bottom surface and sidewall is found in 18650 form factor battery cells. Prior art battery modules use cooling fins or forced air convection through the sidewall of a battery as the primary heat removal mechanism. It is hindered by the poor thermal conductivity of the cell sidewall and not effective in heat removal. Furthermore, cooling air is warmed as it blows across cells, and the battery cell at the end, closest to the air outlet, has the highest air temperature. Every 10 degrees Celsius higher cell temperature halves the battery operating life, prior art battery pack design often cannot meet operating life requirement without stipulating a cooler air inlet temperature than ambient temperature. Other prior art battery module tries to remove heat from the top/bottom surface of battery cells without a mechanically robust connection. Under high shock and vibration vehicle operation, thermal connection from the top/bottom surface of battery cells to external heat sinks can be broken. Because air is a very good thermal insulator, air gap of 0.1 mm is sufficient to stop the transfer of heat away from the battery cell. If battery cell to mechanical housing design is not robust, cell wall can rub against housing under high shock and vibration vehicle operation and sustain damages. Physically damaged cell will lead to shorter operation life. Physically damaged cells can leak flammable electrolyte into the battery pack, creating a fire hazard. Physically damaged cell can also lead to thermal runaway.
18650 type battery cells have electrodes in opposite ends. Many pouch cell designs also have electrodes in opposite ends. Cells must be connected in series in order to meet the voltage requirement of vehicle applications. Typical requirement of 360-volt system can be met by placing 100 of 3.6-volt cells in series. A battery module design that brings the two electrodes into the same plane facilitates inter-module connection in production and field replacement. Furthermore, a battery module design that allows for serial connection within the module is even better.
Therefore, there is a need for a reliable battery module, in which battery cells are mounted in a mechanically robust manner to prevent cell damage or rupture from vehicle shock and vibration, and have improved heat dissipation capability to prolong operating life, and have high safety factor of preventing battery pack from thermal runaway chain reaction in a vehicle crash. Advantageously, the battery modules can be easily installed and maintained in production or in field repairs.